5. Field of the Invention
The present invention relates to a sewing machine and more particularly relates to an apparatus for detecting/displaying a residual quantity of lower or under thread of a sewing machine.
2. Description of the Prior Art
In a sewing machine, in general, an under-thread bobbin on which an under or lower thread is wound is contained inside a bobbin carrier. The bobbin carrier is mounted inside a rotating hook in an inserted manner, while being prevented from rotating by a detaining member during sewing operation. On the other hand, the hook is rotated in synchronism with rotation of an upper driver shaft which vertically reciprocates a machine needle carrying an upper thread, and catches the upper thread to take around the bobbin carrier so that the upper-thread is interlocked with the lower thread as the needle goes up, and thus lock stitches are formed.
In a conventional sewing machines, for example, as shwon in FIG. 10 and as disclosed in JP-A No. 58-12684 (cores. to U.S. Pat. No. 4,333,411), a detecting means for detecting a residual quantity of the under thread wound on the bobbin is constructed of a light emitting device and light receiving device both of which are disposed outside the rotating hook so that light rays emitted from the light emitting device are oriented to the light receiving device is a direction perpendicular to an axis of the under-thread bobbin to impinge on the light receiving device, whereby an amount of light passed through the bobbin is detected by the light receiving device to determine a residual quantity of the under thread still wound on the bobbin.
However, the conventional detecting means of the above construction has a problem of a poor detection reliability in case that if the light range is shorter than the axis of the bobbin, only a part of the residual quantity of the under thread wound on the bobbin is detected.
If the light range is made equal to the axis of the under-thread bobbin, it is required to provide considerably large openings on the bobbin carrier and the rotating hook for permitting the enlarged light range to pass through each of the bobbin carrier and the rotating hook.
However, such large openings formed on each of the bobbin carrier and the rotating hook will reduce the structural strength of the bobbin carrier and the rotating hook. In addition, thread dust and cloth dust tend to accumulate in the vicinity of these openings, and decrease detection accuracy of the detecting means.
Further, it is also known that, for example as disclosed in JP-U1 No. 55-43897 (corres. to U.S. Pat. No. 4,237,807), in a vertically-arranged rotating-hook type sewing machine, the light rays emitted from the light emitted device of the detecting means of the sewing machine are oriented in a direction parallel to an axis of the under thread bobbin.
In this type of the sewing machine, it is necessary to form in a portion of the rotating hook an opening for permitting the light rays to pass through the rotating hook in a direction parallel to the axis of the under-thread bobbin. However in this case, since the light rays are permitted to pass through the opening of the rotating hook only when the opening is aligned with a path of the light rays parallel to the axis of the under-thread bobbin, it is necessary to take a timing of such alignment. This will make the detecting control more complex.
Furthermore, in this case, it is very difficult to provide the light emitting device and the light receiving device around the rotating hook, and the detecting means becomes more complex in construction.